1. Field of Invention
The present invention relates to band-gap reference circuits and in particular to low supply voltage, low spreading and high Power Supply Ripple Rejection Ratio band-gap reference circuits.
2. Description of Related Art
Band-gap reference circuits provide a voltage essentially independent from the operating temperature, supply voltage, and output current. The temperature dependence of transistor characteristics is detrimental to this design goal. In particular, Vbe, the base-emitter voltage of bipolar junction transistors typically has a negative temperature coefficient, or “tempco”. This means that the derivative of Vbe with respect to the temperature, T is negative: dVbe/dT<0. This negative tempco can be compensated by creating an output voltage, which is the sum of Vbe and a compensating Vpt voltage:Vbg=Vbe+Vpt  (1)
Here Vbe is the emitter-base voltage of the forward biased bipolar transistor junction, and Vpt is the PTAT (Proportional To Absolute Temperature) voltage. Visibly, if a Vpt is generated with a temperature coefficient, which is equal in magnitude to the negative tempco of Vbe, but opposite in sign, the sum of these two voltages becomes essentially temperature independent. Since this temperature-independence is achieved by applying voltages close to the band-gap of silicon, these circuits are often termed “band-gap” reference circuits. Correspondingly, the sum of the two voltages is denoted by Vbg.
The dependence of the band-gap reference voltage on the supply voltage is characterized by the ripple rejection ratio. The higher the ripple rejection ratio, the weaker the dependence on the supply voltage.
The dependence of the band-gap reference voltage on the load, or output current, is characterized by the load dependence, or loop gain. The higher the loop gain, the weaker the dependence on the load.
Existing designs of band-gap reference circuits either require a high supply voltage for proper operation, or if they operate at low supply voltages such as 1.3–1.4V, the ripple rejection ratio or load gain of these circuits is limited to the range of about 30 dB to 40 dB